Pituitary adenylate cyclase activating polypeptide (PACAP) was originally isolated from the hypothalamus based on its ability to stimulate adenylate cyclase in rat pituitary cell cultures. PACAP exists in two amidated forms with 38 (PACAP38) and 27 (PACAP27) amino acids, and PACAP38 is the major form in tissues. PACAP is a pleiotropic peptide that acts as a hypophysiotropic hormone, neurotransmitter and neuromodulator. More important, it functions as a neurotrophic factor that regulates neuronal development in the embryonic brain, and prevents neuronal damage in the adult brain. PACAP, at subpicomolar concentrations, is able to completely suppress the cell death induced by gp120 in neuron/glia co-cultures. Although PACAP exerts a direct neurotrophic effect on neuron cultures, nano- or subnanomolar concentrations of the peptide are required for this effect. Our study showed that intravenously administered PACAP38 prevented loss of pyramidal cells in the CA1 field of the hippocampus following global ischemia and reduced the infarct volume due to focal ischemia following the middle cerebral artery occlusion in the rat, even when the treatment was delayed. Although PACAP can enter the brain from the blood across the blood-brain barrier with an efficiency greater than other peptides and even morphine, the level which can be reached in the brain after systemic administration does not seem to reach nanomolar concentrations, but only subpicomolar concentrations. Based on these findings, we have hypothesized that te neuroprotective effect of subpicomolar concentrations of PACAP requires activation of astrocytes, and possibly microglia, which express the specific PACAP receptor (PAC1-R). It has been assumed that an interaction between subpicomolar PACAP and a PAC1-R variant triggers an intracellular signaling cascade that leads to an increased expression of a neurotrophic factos(s). We propose to investigate these hypotheses using various in vitro models, and determine the signaling pathway as well as the key effector molecules involved in the neuroprotective action of PACAP. A better understanding of the mechanism of the neuroprotective action of PACAP38 will help to maximize the therapeutic efficacy of systemic administration of PACAP38 for neuronal damage resulting from stroke and other CNS disorders.